Jet engines are reaction engines that discharge fast moving jets which generate thrust by jet propulsion in accordance with Newton's laws of motion. In these propulsion systems, fluid enters the system at speed V0 with a mass flow of dm/dt and exits at speed Ve. Mass is added to the outflow at a rate dmf/dt. Examples of jet engines include turbojets, turbofans, rockets, ramjets, pulse jets, and propeller-driven systems.
Marine propulsion is the mechanism or system used to generate thrust to move a ship, a boat, or other marine vehicle across and/or through water. While paddles and sails are still used on some smaller boats, most modern ships and other marine vehicles are propelled by mechanical systems consisting a motor or engine turning a propeller, or in jet drives, an impeller. Steam engines were the first mechanical engines used in marine propulsion, but have mostly been replaced by two-stroke or four-stroke diesel engines, outboard motors, and gas turbine engines on faster ships. Nuclear reactors producing steam are used to propel warships and icebreakers, and there have been attempts to utilize them to power commercial vessels. Electric motors have been used on submarines and electric boats and have been proposed for energy-efficient propulsion. Recent development in liquified natural gas (LNG) fueled engines are gaining recognition for their low emissions and cost advantages.